1. Field of the Invention
The present invention relates to a seat back structure of vehicle seat provided with a headrest. In particular, the invention is directed to a seat back structure of vehicle seat so designed that, when a rear-end collision occurs, a seat occupants head is brought to a point adjacent to a headrest mounted on the seat back, so that the head is quickly received by the headrest with a minimum impact, to thereby protect a neck of the seat occupant against damage, such as a whiplash injury.
2. Description of Prior Art
For example, the Japanese Laid-Open Patent Publication No. 2007-186103 (JP 2007-186103 A) and the U.S. Pat. No. 7,540,562 disclose a seat back structure with a movable headrest, which is designed for protecting a seat occupant's head against damage in the case of a rear-end collision. In brief, when a rear-end collision occurs, the body of seat occupant is naturally displaced backwardly of the seat under a backward inertia, which in turn applies a corresponding load (or impact load) to a pressure receiving element provided in the seat back. Thus, the pressure receiving element is moved backwardly to cause a headrest to quickly displace forwardly and upwardly to a point adjacent to a head of the seat occupant.
According to this kind of emergency active headrest, a biasing means, or a tension spring for example, is provided between the pressure receiving element and the headrest. Normally, the headrest is biasingly retained by such biasing means toward a lower home position where it receives a head of seat occupant in an ordinary manner. The biasing means in this headrest system, or the tension spring for example, has a predetermined biasing force sufficient to withstand a weight of the seat occupant's upper body portion as well as a normal load applied from that upper body portion, unless an impact load is applied thereto in a rear-end collision case by a greater force than the seat occupants upper body portion weight and the normal load. Hence, only when such great impact load is applied to the seat back, the pressure receiving element is moved backwardly against the biasing force of the tension spring, so that the headrest is displaced from the lower headrest home position in forward and upward directions towards a predetermined upwardly displaced point of headrest.
With such arrangement, upon occurrence of rear-end collision, the headrest is quickly displaced toward a seat occupant' head being concurrently displaced forwardly, thereby insuring to keep a smallest possible space between the seat occupants head and headrest, and therefore it is possible to effectively reduce a degree of subsequent backward inclination of the seat occupant's head, wherein such subsequent backward inclination of seat occupant's head is quickly followed by forward displacement of that particular head under a backward inertia of the seat occupant, as is known. Accordingly, in this prior art, an abrupt backward inclination of the seat occupant's head can be prevented assuredly, and it is possible to minimize injury of his or her neck portion or whiplash injury.
According to both of the foregoing two prior arts (JP 2607-186103 A and U.S. Pat. No. 7,540,562 B2), a substantially flat support member is provided in the seat back frame so as to be disposed anteriorly of the afore-said pressure receiving element in a backwardly inclined manner, such that the upper end of the support member is situated more backwardly than the lower end thereof. This support member is made of a resilient material and adapted for resiliently receiving a back portion of seat occupant via a padding placed on the frontal surface of that support member. Further, in both of the two prior arts, a pair of rotating links are rotatably secured to the two lateral frame members of the seat back frame, respectively, and a support shaft, on which a headrest is fixed, is connected between those two rotating links and also operatively connected the pressure receiving element through transmission and link elements. When a rear-end collision occurs, the upper body portion or back portion of seat occupant is abruptly displaced backwardly under inertia to the seat back and strongly pressed thereagainst, so that a corresponding excessive great load is applied to the support member and transmitted to the pressure receiving element in the seat back, and the pressure receiving element is thereby displaced backwardly. With such backward displacement of the pressure receiving element, the transmission and link elements operatively connected with that pressure receiving element are actuated to cause rotation of both two rotating links, thereby causing the support shaft integral with the two rotating links to rotate upwardly, so that the headrest fixed on the support shaft is quickly displaced upwardly from the lower home position to the predetermined upwardly displaced point. Hence, the headrest is immediately brought to a point adjacent to the seat occupant's head, thereby reducing a space between the headrest and the seat occupant's head to the extent that the previously discussed abrupt backward inclination of seat occupant's head can be prevented, and therefore, an injury of his or her neck portion or whiplash injury can be minimized.
However, the above-described prior-art arrangement of seat back and active headrest has been with the problem that the seat back structure thereof is very complicated with, many mechanisms incorporated therein, hence requiring troublesome and time-consuming processes for assembling the seat back, with highly increasing costs involved, and that the provision of the pressure receiving element not only limits a cushioning degree of the seat back, which does not provide a deep resilient support touch to a seat occupant's back portion, but also makes the seat occupant feel a hard objectionable touch at his or her back portion.